The present invention relates to automotive wheel alignment apparatus and, more particularly, to optical apparatus for measuring the toe-in and toe-out of the front wheels of automotive vehicles. (As used herein, the term "wheel" refers to the combination of a metal wheel and a tire thereon.)
Such apparatus is disclosed, for example, in U.S. Pat. No. 3,953,134 issued to A. V. Appel et al. on Apr. 27, 1976, and U.S. Pat. No. 3,393,455 issued to C. W. MacMillan on July 23, 1968. Such prior art apparatus typically includes a pair of support arms respectively supported from the wheel hubs and extending forwardly of the wheels, either or both of said arms being provided at the forward end thereof with a light beam projector which projects a beam onto a screen supported by the opposite arm. Such prior art systems are broadly divided into single-projector systems wherein the screen and projector are supported from one arm and the other arm supports a mirror for reflecting the light beam, and two-projector systems wherein each arm carries its own projector and screen. In either type of system, the screen is disposed substantially parallel to the longitudinal axis of the support arm, i.e., substantially perpendicular to the axis of the associated wheel.
In using the prior art systems, the vehicle front wheels are typically driven onto a ramp which is either elevated above the garage floor or is suspended over a pit or well in the garage floor to permit an operator to position himself beneath and between the vehicle front wheels for access to the tie rods for making the necessary toe adjustments. Because of the position of the screens forward of the wheels and their orientation perpendicular to the wheel axes, it is difficult for the operator to read accurately the toe measurements on the screen from the toe adjusting position between the wheels, since the screens are disposed at relatively shallow angles to his lines of sight. The difficulty in reading the scales on the screens accurately is further aggravated by the fact that the specified toe dimensions for each wheel are typically only fractions of an inch and, likewise, the individual scale divisions on the screens are also separated by only fractions of an inch and are quite small. Accordingly, it is normally necessary for the operator to repeatedly move forwardly of the wheels to read the screen scale and then back to the adjustment position to make the necessary adjustment of the wheel tie rods until the screen indicates that the wheel has been set to the desired toe angle.